Electrodes with improved adhesion between activator and collector and methods of making the same

ABSTRACT

The present invention relates to electrodes consisting of an electrode activator and a metallic collector whose adhesion between the activator and the collector is improved. The electrode activator&#39;s binder is made of a fluoroplastic grafted with at least one acrylic polymer.

TECHNICAL FIELD

This invention relates to electrodes which can be used in batteries andcells, such as lithium-ion batteries and cells and the production methodof the said electrodes.

PRIOR ART

There is a great demand for small secondary cells having a high capacityand a long life in portable instruments such as portable telephone sets,video cameras and notebook type personal computers, etc. Lithium-ioncells are future-expected secondary cells.

Known anode activator substances in the lithium-ion cell consistgenerally of carbonaceous materials such as coke or graphite into whichthe lithium ions can be doped or released reversibly (JP-A 62-90863).Usually, a powder of carbonaceous material is mixed with a suitableamount of a binder and is kneaded with a solvent in order to prepare apaste. A collector is then coated with the paste and is dried andcompacted to obtain the anode.

Known cathode activator substances in the lithium-ion cell consistgenerally of transition metals oxides such as manganese oxide andvanadium oxide, sulfides of transition metals such as iron sulfide andtitanium sulfide, or composite compounds of the above substances andlithium such as composite oxides of lithium and cobalt, composite oxidesof lithium, cobalt and nickel, composite oxides of lithium andmanganese. The cathode activator substance also is mixed with anelectro-conductive substance (usually carbon) and a suitable amount of abinder and is kneaded with a solvent in order to prepare a paste whichis then applied to a collector and is dried and compacted to obtain acathode.

The binder for secondary battery must have a high resistance to liquidswhich are often used as electrolytes and to active species generated bythe electrochemical reactions and also to solvents which are carried outduring the manufacture of the batteries and cells. A binder whichsatisfies the above requirements is polyvinylidenefluoride (PVDF) resin. However PVDF resins and fluorinated resins in general have inherentlypoor adhesion to metals, so that the activator substance separateseasily from the metallic collector for both cathode and anode and itresults that an inferior cycle property of the lithium-ion cell.

JP-A-5-6766 has proposed to roughen the collectors' surface in order toincrease the anchoring effect of the fluorinated resins. However, asufficient adhesion cannot be obtained in carrying out this technique.

A copolymer of vinylidenefluoride (VF2) and a carboxyl group-containingmonomer has been proposed in JP-A-6-172452. This copolymer, however, isdifficult to produce industrially.

DISCLOSURE OF THE INVENTION

The present invention provides electrodes for batteries and cells whoseadhesion between the electrode activator and the collector is improvedso as the cycle property of the cells.

MEANS TO SOLVE THE PROBLEM

The present invention provides electrodes for cells having a layer of anelectrode-forming substance comprising an electrode activator and abinder which is coated and/or bonded on a surface of a metalliccollector, characterized in that the binder is a fluoroplastic to whichat least one acrylic polymer is bonded, the monomer units of the-saidacrylic polymer(s) consisting mainly of at least one monomer unitselected from esters of acrylic acid and/or methacrylic acid.

In the grafted fluoroplastic according to the invention, the content ofthe acrylic polymer is 0.1 to 20% by weight, preferably 0.2 to 20% byweight, more preferably 0.3 to 5% by weight of the graftedfluoroplastic. If the content is less than 0,1% by weight, the adhesionbetween the electrode activator and the collector is poor; if thecontent is more than 20% by weight, the binder's resistance becomes poorand an important swelling caused by the contact with the organicsolvents used as electrolytes (for instance ethylenecarbonate,propylenecarbonate, dimethyl carbonate, diethyl carbonate etc) isobserved. Consequently both low and high content of acrylic polymer havea bad influence on the performances of the electrodes and of thesecondary cells. These drawbacks become particularly serious when thetemperature is higher than 50° C.

The collector for both electrodes (anode and cathode) may be a metalfoil, a metal mesh, a three-dimensional porous block or the like and ispreferably made of a metal which does not easily produce an alloy withlithium as iron, nickel, cobalt, copper, titanium, vanadium, chromiumand manganese or one of their alloys.

The anode activator substances can be any materials which permit dopingand releasing of lithium ions and are generally carbonaceous materialsincluding cokes such as petroleum cokes and carbon cokes, carbon blackssuch as acetylene black, graphite, fibrous carbon, activated carbon,carbon fibers and sintered articles obtained from organic high polymersby burning the organic high polymer in a non-oxidative atmosphere. Metaloxides as copper oxide can also be added to the anode activatorsubstance.

The cathode activator substances can be usual known ones as disclosedabove. Some electro-conductive materials can also be incorporated in thecathode activator substance.

The fluoroplastic may be polytetrafluoroethylene, polyvinyl fluoride,polytrifluoroethylene, polychlorotrifluoroethylene, copolymer ofvinylidenefluoride-chlorotrifluoroethylene, copolymer of ethylene andtetrafluoroethylene, copolymer of tetrafluoroethylene and hexafluoropropylene and polyvinylidenefluoride (PVDF). Among them, PVDF ispreferably used because of its high resistance to solvents generallyused in cells and active species produced and also because of its goodsolubility in N-methylpyrolidone which is generally used during themanufacture of batteries and cells.

For the present invention, PVDF means homopolymers of vinylidenefluoride(VF2) and copolymers of VF2 and at least another fluorinated comonomerpreferably chosen among tetrafluoroethylene, hexafluoropropylene,trifluoroethylene and/or chlorotrifluoroethylene that can be used aloneor in combination. The amount of VF2 is from 40 to 95% by weight andpreferably from 70 to 95%. The preferred PVDFs according to the presentinvention have a melt flow index (MFI) of 0.01 to 300 g/10 min at 230°C. under a load of 2.16 kg.

The main monomer units of the acrylic polymer are, as said above, theesters of acrylic acid and/or methacrylic acid; they may be alkyl estersof acrylic acid or methacrylic acid as methylacrylate, ethylacrylate,methylmethacrylate, ethylmethacrylate and butylmethacrylate. The amountof these monomers in the acrylic polymer(s) is preferably more than 80%by weight of the acrylic polymer.

Preferably, the acrylic polymers have 0.2 to 20 parts by weight,preferably 1 to 10 parts by weight of carboxyl group or carboxylanhydride groups. The monomers containing carboxyl group or carboxylanhydride group may be unsaturated carboxylic acids as acrylic acid,methacrylic acid, crotonic acid, maleic acid, fumaric acid, alkenylsuccinic acid, acrylamideglycolic acid and monoallyl-1,2-cyclohexanedicarbonate and unsaturated carboxylic anhydride such asmaleic anhydride and alkenyl succinic anhydride.

The acrylic polymers as defined above can be grafted to thefluoroplastic by graft reaction with the help of at least a peroxideeffected after the polymerization of the fluoroplastic, by means ofradiation or by polymerization of the fluorinated monomers (of the-saidfluoroplastic) in the presence of the acrylic polymer(s).

The graft reaction with peroxide(s) is carried out in heating a mixtureof the acrylic polymer and the fluoroplastic in the presence of theperoxide(s). The reaction can be carried out in molten condition or in asolvent. If a solvent is used, the acryl polymer, the PVDF and theperoxide(s) are both dissolved in a solvent and the resulting solutionis heated at a temperature to which the peroxide decomposessatisfactorily. A suitable amount of peroxide(s) is typically about 0.5to 10% by weight to the fluoroplastic. The resulting solution can beapplied directly onto a collector to produce the electrode. The graftcopolymer can be extracted from the solution by a reprecipitationtechnique, can be optionally purified by washing with a suitable solventand used as a binder for the anode and/or the cathode.

Any known peroxides can normally be used as peroxyketals,alkylhydroperoxides, dialkylperoxides, alkylperoxyesters,dialkylperoxides, peroxydicarbonates and peroxyesters.

In the present invention, the graft reaction can be carried outsimultaneously with the electrode's manufacture. When the graft reactionoccurs with the help of peroxid(s), a slurry can be prepared by kneadingpredetermined amounts of electrode activator, of fluoroplastic,preferably PVDF as a binder, of the above acrylic polymer andperoxide(s) in the presence of a solvent. The resulting slurry is coatedon a surface of the collector, dried and then press-moulded into anelectrode. The slurry is heated before or after its application to thecollector according to the peroxide(s) 's nature.

The binder is preferably added in a range from 1 to 30 parts, preferably3 to 15 parts by weight to 100 parts by weight of the electrodeactivator. Other additives such as electro-conductive agents (copperoxide) also can be added to the electrode-forming substance.

The solvents used during the graft reaction with peroxide(s) or thesolvents used to prepare the slurry to be coated on the collector can bewater or organic solvents such as N-methylpyrolidone, N,N-dimethylformamide, tetrahydrofuran, dimethyl acetoamide, dimethylsulfoxide, hexamethylsulfolamide, tetramethylurea, acetone andmethylethyl ketone. These solvents can be used alone or in combination.Among them, N-methylpyrolidone is preferably used. If necessary, adispersant can be also be used. Nonionic dispersants are preferred.

ADVANTAGES OF THE INVENTION

The present invention provides electrodes whose adhesion between theelectrode activator and the collector is improved. When these electrodesare used in batteries, the capacity of discharge is not deterioratedafter repeated charge-discharge cycles. The present invention is usefulparticularly in lithium-ion cells.

MEANS TO SOLVE THE INVENTION EXAMPLES Preparation Example 1

10% by weight of a PVDF homopolymer (sold by Elf Atochem under Kynar®500, MFI: 0.03 g/10 min at 230° C. under a load of 2.16 kg), 0.2% byweight of an acrylic copolymer (MFI: 2.4 g/10 min at 230° C. under aload of 3.8 kg; comprising 100 parts by weight of methylmethacrylate and10 parts by weight of maleic anhydride), and 0.5% by weight ofbenzoylperoxide were dissolved in N-methylpyrolidone. The solution washeated at 120° C. for 30 min and then was poured into methanol tocollect the polymer which has precipitated.

The resulting polymer was extracted with chloroform by refluxing thepolymer for 6 hours in a Soxhlet extractor to confirm that the acryliccopolymer is grafted to the PVDF. A film prepared with the extractedpolymer was examined by IR spectroscopy; a clear peak at 1740 cm⁻¹ wasfound that corresponds to the absorption caused by the carbonyl groups.

Preparation Example 2

A grafted PVDF was prepared with the same procedure as in PreparationExample 1 but the PVDF homopolymer was changed to a copolymer of VF2(90% by weight) and hexafluoropropylene (10% by weight) sold by ElfAtochem under Kynar®2800 (MFI: 0.2 g/10 min at 230° C. under a load of2.16 kg) and benzoylperoxide was changed to t-butylperoxybenzoate.

Preparation Example 3

9% by weight of the PVDF of Preparation Example 1, 1% by weight of thecopolymer of Preparation Example 2, 0.2% by weight of the same acrylicpolymer of Preparation Example 1 and 0.5% by weight of benzoylperoxidewere dissolved in N-methylpyrolidone; the solution was then heated at120° C. for 30 minutes and then was poured into methanol to collect thepolymer which has precipitated.

Example 1

8 parts by weight of the grafted PVDF copolymer of Preparation Example 1(binder) was dissolved in N-methylpyrolidone and 90 parts by weight ofcoal pitch coke crushed in a ball mill as anode activator carrier wasadded to the solution in order to obtain a slurry (paste). The slurrywas then coated on both sides of a copper foil of 20 μ thickness, driedat 120° C. under reduced pressure and then press-moulded to obtain ananode of 145 μ thickness and of 20 mm width.

In order to prepare a cathode, 92 parts by weight of LiCoO₂ as cathodeactivator and 6 parts of graphite as electro-conductive additive weredispersed in a solution of N-methylpyrolidone in which 8 parts by weightof the same binder that was used for the preparation of the anode wasdissolved to obtain a slurry (paste). The slurry was coated on bothsides of an aluminum foil of 20 μ thickness, dried at 120° C. underreduced pressure and then press-moulded to obtain anode of 175 μthickness and of 20 mm width.

A good adhesion between these electrodes and the electrode activator wasnoted: the electrode activators remain on the electrode surfaces whenthe electrode activators deposited on the electrodes were peeled off bya cutter-knife.

The resulting cathode and anode were laminated alternately through afilm of porous polypropylene of 25 μ thickness as separator to form alaminate of separator/cathode/separator/anode/separator which was woundspirally to obtain a cylindrical electrode assembly. After lead wireswere attached to respective electrodes, the electrode assembly waspacked in a stainless container into which an electrolyte was poured Theelectrolyte is 1 M solution of LiPF₆ dissolved in an equivolumic mixtureof propylene carbonate and 1, 2-dimethoxyethane.

In the charge-discharge test, the battery was charged with a currentdensity of 30 mA / 1 of carbon to 4.1 V and then was discharged with thesame current to 2.5 V. The same charge-discharge operation was repeatedto evaluate the capacity of discharge. The results revealed that thecapacity of discharge after 100 cycles was 90% of a value of 10th cycle.

Example 2

8% by weight of the PVDF used in Preparation Example 1 (Kynar500), 0.1%by weight of the acrylic polymer used in Preparation Example 1 (binder)and 0.2 parts by weight of diisopropylperoxydicarbonate were dissolvedin N-methylpyrolidone. 90 parts by weight of coal pitch coke crushed ina ball mill as anode activator carrier was added to the solution toobtain a slurry (paste). The slurry was heated in a closed system toprevent solvent evaporation at 80° C. for 30 minutes and then was coatedon both sides of a copper foil of 20 μ thickness, dried at 120° C. underreduced pressure and then press-moulded to obtain an anode of 140 μthickness and of 20 mm width.

In order to prepare a cathode, 92 parts by weight of LiCO₂ as cathodeactivator and 6 parts of graphite as electro-conductive additive weredispersed in a solution of N-methylpyrolidone in which, as binder, 8parts by weight of PVDF, 0.1 parts by weight of the acrylic polymer asdefined above and and 0.2 parts by weight ofdiisopropylperoxydicarbonate were dissolved to obtain a slurry (paste).The slurry was heated in a closed system to prevent solvent evaporationat 80° C. for 30 minutes and then was coated on both sides of a aluminumfoil of 20 μ thickness, dried at 120° C. under reduced pressure and thenpress-moulded to obtain a cathode of 160 μ thickness and of 20 mm width.

A good adhesion between these electrodes and the electrode activator wasnoted: each electrode activator remains on the electrode surfaces whenthe electrode activators deposited on the electrodes were peeled off bya cutter-knife. A cell was manufactured by the same method as in Example1 and the same charge-discharge test was effected. The results revealedthat the capacity of discharge after 150 cycles was 93% of a value of10th cycle.

Example 3

The procedure of Example 1 was repeated but the grafted PVDF ofPreparation Example 2 was used.

A good adhesion between these electrodes and the electrode activator wasnoted: the electrode activator remains on the electrode surfaces whenthe electrode activators deposited on the electrodes were peeled off bya cutter-knife. A cell was manufactured by the same method as in Example1 and the same charge-discharge test was effected. The results revealedthat the capacity of discharge after 150 cycles was 92% of a value of10th cycle.

Example 4

The procedure of Example 1 was repeated but the grafted PVDF ofPreparation Example 3 was used.

A good adhesion between these electrodes and the electrode activator wasnoted: the electrode activator remains on the electrode surfaces whenthe electrode activators deposited on the electrodes were peeled off bya cutter-knife. A cell was manufactured by the same method as Example 1and the same charge-discharge test was effected. Results revealed thatthe capacity of discharge after 150 cycles was 95% of a value of 10thcycle.

Comparative Example 1

The same procedure as Example 1 was repeated but PVDF Kynar500 was used.

When carbon pitch coke deposited on electrode was peeled off by acutter-knife, substantially no electrode activator remains on a surfaceof the copper electrode. A cell was prepared by using the resultingelectrode in the same manner as Example 1 to find that the capacity ofdischarge after 100 cycles was 50% of a value of 10th cycle.

Comparative Example 2

The same procedure as Example 2 was repeated but no peroxide was addedto the slurry for the manufacture of both anode and cathode with thesame procedure as in Example 1.

When carbon pitch coke deposited on the electrode was peeled off by acutter-knife, it was recognized that a substantial part of the electrodeactivator remains on a surface of copper electrode. A cell was preparedby using the resulting electrode in the same manner as Example 1. Itscapacity of discharge after 150 cycles was 55% of a value of 10th cycle.

What is claimed is:
 1. Electrode for cell comprising a layer of anelectrode-forming substance comprising an electrode activator and abinder coated or bonded to a surface of a collector, characterized inthat the binder is a fluoroplastic grafted with at least one acrylpolymer consisting mainly of at least one monomer unit selected fromesters of acrylic acid and/or methacrylic acid and that the content ofsaid acryl polymer is 0.1 to 20% by weight of said binder.
 2. Electrodeaccording to claim 1, wherein said fluoroplastic is apolyvinylidenefluoride resin.
 3. Electrode according to claim 2, whereinthe polyvinylidenefluoride resin is a polyinylidenefluoride homopolymer.4. Electrode according to claim 2, wherein the polyvinylidenefluorideresin is a copolymer of vinylidenefluoride and at least one monomerselected from the group comprising tetrafluoroethylene,hexafluoropropylene, trifluoroethylene and/or chlorotrifluoroethylene,the proportion of vinyidenefluoride in the copolymer being more than 40%by weight.
 5. Electrode according to claim 2, wherein thepolyvinylidenefluoride resin is a mixture of a polyvinylidenefluoridehomopolymer and a copolymer of vinylidenefluoride and at least onemonomer selected from the group comprising tetrafluoroethylene,hexafluoropropylene, trifluoroethylene and/or chlorotrifluoroethylene,the proportion of vinylidenefluoride in the copolymer being from 50 to95% by weight.
 6. Electrode according to claim 1, wherein the acrylicpolymer comprises 0.5 to 20% by weight of at least one monomer havingcarboxylic acid and/or carboxylic anhydride group.
 7. The electrodeaccording to claim 1, wherein the content of said acrylic polymer is 0.3to 5% by weight.
 8. A method for producing an electrode for cellcomprising a layer of an electrode-forming substance comprising anelectrode activator and a binder coated or bonded to a surface of acollector, wherein the binder is a fluoroplastic grafted with at leastone acryl polymer consisting mainly of at least one monomer unitselected from esters of acrylic acid and/or methacrylic acid and thatthe content of said acryl polymer is 0.1 to 20% by weight of saidbinder, comprising the steps of: (a) obtaining the binder by graftreaction of the acrylic polymer(s) and the fluoroplastic in the presenceof peroxide(s); (b) kneading the binder with the electrode activator inthe presence of a solvent to prepare a slurry; (c) applying the slurryonto the surface of a collector; and (d) drying the slurry applied instep (c).
 9. A method for producing an electrode for cell comprising alayer of an electrode-forming substance comprising an electrodeactivator and a binder coated or bonded to a surface of a collector,wherein the binder is a fluoroplastic grafted with one or more acrylpolymers consisting mainly of at least one monomer unit selected fromesters of acrylic acid and/or methacrylic acid and that the content ofsaid acryl polymers is 0.1 to 20% by weight of said binder comprisingthe steps of: (a) preparing a slurry from fluorinated monomers,peroxide(s), the acrylic polymer(s) and a solvent; (b) heating theslurry to a temperature suitable for the polymerization and the graftreactions of the fluoroplastic, and (c) coating the slurry obtained instep (a) onto a collector.
 10. A method for producing an electrode forcell comprising a layer of an electrode-forming substance comprising anelectrode activator and a binder coated or bonded to a surface of acollector, wherein the binder is a fluoroplastic grafted with one ormore acryl polymers consisting mainly of at least one monomer unitselected from esters of acrylic acid and/or methacrylic acid and thatthe content of said acryl polymer is 0.1 to 20% by weight of said bindercomprising the steps of: (a) preparing a slurry of fluorinated monomers,peroxide(s). the acrylic polymer(s) and a solvent; (b) coating theslurry obtained in step (a) onto a collector; and (c) heating theslurry-coated collector obtained in step (b) to a temperature suitablefor polymerization and graft reaction of the fluoroplastic.
 11. A methodfor producing an electrode for cell comprising a layer of anelectrode-forming substance comprising an electrode activator and abinder coated or bonded to a surface of a collector, wherein the binderis a fluoroplastic grafted with one or more acryl polymers consistingmainly of at least one monomer unit selected from esters of acrylic acidand/or methacrylic acid and that the content of said acryl polymer is0.1 to 20% by weight of said binder comprising the step of: (a)obtaining the binder using means of radiation to cause graft reaction ofacrylic polymer(s) and the fluoroplastic.